1. Field of the Invention
The invention relates to a polarization element of a light irradiation device for optical alignment of liquid crystals in which the alignment layer of a liquid crystal cell element is irradiated with polarized light. The invention relates especially to a polarization element which is suitable for optical alignment of a wide-view film.
2. Description of Related Art
A liquid crystal cell element is produced as follows:
An alignment layer formed on the surface of a transparent substrate is treated for alignment of the liquid crystals in a desired direction (alignment treatment).
Two of these transparent substrates are cemented to one another such that the alignment layers are located on the inside and a gap with a stipulated distance is maintained between the two substrates.
Liquid crystals are injected into this gap. For the above described alignment treatment of the alignment layer of a liquid crystal cell element, there is a technique which is called xe2x80x9coptical alignmentxe2x80x9d. Here, an alignment layer is irradiated with polarized light and exposed.
An irradiation device for polarized light for optical alignment is disclosed, for example, from the Japanese patent disclosure document HEI 10-90684.
Recently the above described irradiation device for polarized light is being used more and more often to produce a wide-view film, besides for production of the above described liquid crystal cell element. A wide-view film is produced as follows:
Liquid crystals which can be cured using UV light are applied to a base film.
Liquid crystal molecules are aligned in a certain direction.
Afterwards the liquid crystals are cured by UV irradiation so that the direction of the liquid crystal molecules is fixed.
By cementing a wide-view film onto a liquid crystal cell, a reduction of the image quality can be equalized, i.e., the field of view of LCDs can be enhanced.
Conventionally, the above described process of xe2x80x9calignment of the liquid crystal molecules in a certain directionxe2x80x9d is carried out by rubbing. But recently, it is being done more and more often using the above described irradiation with polarized light and by optical alignment.
FIG. 7 shows the arrangement of an irradiation device for polarized light which optically aligns a wide-view film according to the example. In the figure, a discharge lamp 1, such as a super high pressure mercury lamp or the like, is located in an oval focusing mirror 2, and directs light from the lamp to a first planar mirror 3 from which it passes through a polarization element 8, an integrator lens 4, and a shutter 5 to a second planar mirror which deflects the light to a collimation lens 7. These components make up the irradiation device for polarized light.
In the polarization element 8, several glass plates are inclined by the Brewster angle with respect to the optical axis of the incident light beam and are located parallel to one another at intervals. These glass plates are combined with one another in a V-shape and are arranged such that the light exit side becomes convex.
When a polarization element is used which is made of glass plates which have been combined in a V-shape, the advantages are the following:
The size of the entire polarization element (the size in the lengthwise direction with respect to the optical axis) can be reduced more than in the case in which the glass plates are not combined in a V-shape.
The size of the individual glass plate becomes smaller.
The production costs per glass plate become smaller.
Two directions for the arrangement of the polarization element formed of glass plates which have been combined in a V-shape can be imagined, i.e., the direction in which the exit side is convex, and the direction in which the incidence side is convex. FIG. 7 shows a polarization element in which the exit side is convex (in which the light is incident from the direction in which the glass plates form a xe2x80x9cvalleyxe2x80x9d).
The number of glass plates used for the polarization element is not limited to the number in the drawings, but it can be set suitably according to the desired illuminance on a surface to be irradiated and according to the extinction ratio.
In the figure, the light which is emitted from the discharge lamp 1 and which contains UV radiation is focused by the oval focusing mirror 2, reflected by the first plane mirror 3 and is incident in the polarization element 8. The polarization element 8 passes P-polarized light and reflects most of the S-polarized light. The light emerging from the polarization element 8 therefore becomes mainly P-polarized light. The light emerging from the polarization element 8 is incident in the integrator lens 4.
The light emerging from the integrator lens 4 is reflected via the shutter 5 and by the second plane mirror 6, is converted into parallel light by means of the collimation lens 7 and is emitted onto a wide-view film 10 to which liquid crystals which can be cured with UV light have been applied. The concept of xe2x80x9cwide-view film 10xe2x80x9d is defined below as a film to which liquid crystals which can be cured with UV light have been applied.
The wide-view film 10 is wound in the manner of a roll as an endless workpiece with a great length onto a take-off roller 11a. After the wide-view film which has been wound onto the take-off roller 11a has been unwound and irradiated with polarized light, it is wound onto the take-up roller 11b, as is shown in the drawings. During transport of the film 10 the polarized light is emitted onto the film 10 with a pre-set angle.
It is known that the above described wide-view film is also aligned at a poor extinction ratio of the polarized light (for example, P-polarized light to S-polarized light=3:1 to 2:1). Polarized light with this extinction ratio can also be obtained without using an optical element such as a collimator or the like.
In FIG. 7, the polarization element 8 is located between the first plane mirror 3 and the integrator lens 4. But, as shown in FIG. 8, the polarization element 8 can be located on the exit side of the collimation lens 7.
FIG. 9 shows another example of the arrangement of an irradiation device for polarized light for optical alignment of a wide-view film. In this embodiment, a rod-shaped tube lamp 21, for example a high pressure mercury lamp or a metal halide lamp is located at the focal point of a trough-like mirror 22 with a cross section which is a parabola. In the polarization element 8xe2x80x2, as in the polarization element 8 shown in FIG. 7, several glass plates are inclined by the Brewster angle with respect to the optical axis of the incident light beam and are arranged parallel to one another at intervals. These glass plates are combined with one another in a V-shape and are arranged such that the light exit side becomes convex.
The wide-view film 10, as was described above, is an endless workpiece of great length that is wound as a roll onto the take-off roller 11a. After the wide-view film which has been wound onto the take-off roller 11a has been unrolled and irradiated with polarized light, it is wound onto the take-up roller 11b as is shown in the drawings. During transport of the film 10, the polarized light is emitted onto the film 10 at a pre-set angle.
In the irradiation device for polarized light shown in FIG. 9, the directly projected light from the lamp is scattered light. Therefore, light with a non-Brewster angle is incident in the polarization element. Furthermore, there is also light which passes through the polarization element without polarization separation.
However, the light which has been reflected by the trough-shaped mirror 22 is converted essentially into parallel light and is incident with the Brewster angle in the polarization element, by which it is subjected to polarization separation. As a result P-polarized light emerges. Therefore, light emerges from the polarization element which, for the most part, has a large proportion of P-polarized light and which is emitted onto the wide-view film 10.
The reason for using such a rod-shaped lamp is that a wide surface can be irradiated with high illuminance, that the film exposure rate is increased, and that the throughput is improved, compared to the case in which the point light source shown in FIGS. 7 and 8 is used.
If, as was described above, a polarization element is used in which several glass plates are inclined with respect to the optical axis of the incident light beam at the Brewster angle and are arranged spaced parallel to one another, and in which the glass plates are combined with one another in a V-shape, the advantages are the following:
The size of the entire polarization element can be reduced.
The size of the individual glass plate becomes smaller.
Production costs per glass plate become less.
If the polarization element in which the glass plates are combined with one another in a V-shape is used and if this polarization element is arranged such that the light exit side becomes convex, the disadvantages are the following:
FIG. 10 shows a polarization element in which glass plates are combined with one another in a V-shape. Here, four sets of glass plates 8a which have been combined with one another in a V-shape are used, and thus, a polarization element is formed.
The respective glass plate 8a is inclined with respect to the incident light by the Brewster angle and is arranged such that the exit side becomes convex. In FIG. 10, the arrows to the top and bottom and the circles with the black dots in the middle label the polarization directions of the P-polarized light and the S-polarized light. As is shown in the drawings, on the exit side of the polarization element 8, a shadow is formed by the refraction of the glass plates. In fact, 15 sets of glass plates are used. However, in FIG. 10, only four sets of glass plates are represented and the others are omitted.
As is shown in FIG. 10, the light which is incident in the glass plates with the Brewster angle is separated into P-polarized light and S-polarized light. The P-polarized light is passed by the glass plates while the S-polarized light is reflected. The S-polarized light reflected by the surface of the glass plate is incident in the opposite glass plate. Since the light incident in this glass plate from the above described reflected light (of the S-polarized light) does not have the Brewster angle, it passes through the glass plate even if it is S-polarized light.
The S-polarized light which has passed though this glass plate is reflected by the surface of the second glass plate in the same way and transmitted and is likewise reflected by the surface of the third and other glass plates in the same way and transmitted. This repetition ultimately yields S-polarized light which passes through the polarization element. As a result this light becomes xe2x80x9cstray light,xe2x80x9d by which the extinction ratio of the polarized light which emerges from the polarization element is reduced.
An object of the invention is to eliminate the above described defects of the prior art.
More specifically, it is an object of the invention is to prevent the S-polarized light reflected from the surface of a glass plate from being incident in an opposite glass plate, thus to reduce the S-polarized light which passes through the polarization element, and to increase the extinction ratio of the polarized light which emerges from the polarization element.
The above objects and others are achieved in accordance with the invention as follows:
(1) A polarization element is used in which several glass plates are inclined at the Brewster angle with respect to the optical axis of the incident light beam and are located parallel to one another at spaced intervals, and in which these glass plates are combined with one another V-shaped polarization element that is arranged such that the light exit side is convex. On the light incidence side of this polarization element, along the edge line, there is a light shielding component, this edge line being formed by the crossing of the light incidence surfaces of the above-described two sets of V-shaped glass plates. The light shielding component is essentially parallel to the optical axis of the incident light such that the light which is incident in the above described glass plate with the Brewster angle is not hindered. The light shielding component shields the S-polarized light which is reflected by the above described glass plate.
By the arrangement of the above described light shielding component the S-polarized light reflected by the surface of the glass plate is absorbed or reflected by the light shielding plate and is not incident in the opposite glass plate. As a result, the S-polarized light which passes through the polarization element can be diminished. Thus, the adverse effect of the above described reflection light on the extinction ratio can be reduced.
(2) In (1), there is the above described light shielding component for each of the several glass plates which are combined with one another in a V-shape.
In this way, the S-polarized light which is reflected by the several glass plates which are combined with one another in a V-shape can be completely shielded, and thus, the adverse effect of the above described reflection light on the extinction ratio can be reduced even more.
The invention is explained in detail below using several embodiments shown in the drawings.